This invention is directed to methods for treating water containing a oxidizing agent by controlling the addition of a reducing agent to the water as it flows through a conduit or is contained in a vessel, to neutralize the oxidizing agent and control the concentration of the reducing agent in the water after neutralization. More particularly, this invention relates to a method of using oxidation reduction measurement technology to dechlorinate wastewater and control the residual reducing agent concentration in the wastewater prior to discharge from a wastewater treatment plant.
In wastewater treatment systems, the number of harmful bacteria or viruses per unit of wastewater must be reduced to acceptable levels prior to the return of the wastewater to the environment, a process known as disinfection. Disinfection of the wastewater is typically carried out by the introduction of an oxidizing agent into the wastewater as the wastewater enters a contactor in a wastewater treatment plant. As the wastewater flows through the contactor, the oxidizing agent attacks and destroys the harmful bacteria and largely disinfects the wastewater. Prior to the discharge of the wastewater from the contactor into the environment, a reducing agent is added to the wastewater to remove the oxidizing agent because even small amounts of oxidizing agents may be harmful to aquatic life. The amount of reducing agent added is generally a large overdose of the amount necessary to remove the oxidizing agent. Therefore, the water is discharged with a high concentration level of the reducing agent.
It is known that the kill rate on bacteria and viruses can be characterized by the oxidation reduction potential or ORP of the wastewater. The addition of the oxidizing agents raises the ORP of the wastewater, resulting in disinfection. Therefore, an oxidizing agent is added to the wastewater in such an amount so that the wastewater reaches a predetermined ORP level which will result in disinfection. The oxidizing agent typically is chlorine, although bromine, ozone, iodine and the like are also used.
Devices which measure ORP of a fluid are referred to as oxidation/reduction potential or redox probes. In a redox probe, a noble metal such as platinum is used as one electrode. The platinum electrode is exposed to the wastewater. Oxidizing agents present in the wastewater exert an EMF (electromotive force) on the platinum, changing its electrical potential (voltage). The analyzer component of the probe then compares this voltage to a constant-voltage reference electrode. This measurement is displayed as millivolts or concentration in mg/l. The measurement output from the redox probe may be used to control the addition of the oxidizing agent.
Concern over the environmental impact of the oxidizing agent in wastewater has lead to a desire to neutralize the oxidizing agent in the wastewater after disinfection and prior to the discharge of the wastewater into the environment. This neutralization is generally achieved through the addition of a reducing agent to the wastewater. The most prevalent types of reducing agents used are compounds which form sulfite upon introduction into the wastewater such as sodium-bisulfite or soluble metallic sulfite.
It is known that a redox probe may be used to measure low levels of oxidizing agents in the wastewater. The output from the probe has been used to control the addition of a reducing agent when the goal of the neutralization was to maintain a low concentration level of chlorine in the discharge of the wastewater.
Other prior art sensing devices, such as the amperometric and colorimetric analyzers, typically cannot sense the presence of reducing agents in wastewater and are only marginally accurate when trying to measure oxidizing agent residuals near zero. Therefore, when the goal is to completely neutralize the oxidizing agent, these sensing devices may be placed before the sulfite feeder to measure the oxidizing agent content of the wastewater and then an amount of sulfite is added and mixed into the wastewater to neutralize the measured oxidizing agent level. However, by placing the sensing device before the sulfite feeder, the sensing device cannot then determine whether the amount of sulfite added has completely neutralized the oxidizing agent. Therefore, to insure complete neutralization, the amount of sulfite added to the wastewater is generally a large overdose of the stoichiometric amount necessary to neutralize the measured oxidizing agent level.
This overfeeding of sulfite has several drawbacks. Overfeeding adds cost to the neutralization program. Excess sulfite may also pose a health hazard and therefore run afoul of governmental regulations. The excess sulfite may also have a negative impact of the environment by depleting dissolved oxygen which is essential to aquatic life.
It is therefore an object of the present invention to provide a method of controlling the addition of a reducing agent to water flowing through a conduit or contained in a vessel to completely neutralize an oxidizing agent in the water and control the concentration of the reducing agent in the discharge of the water from the conduit or in the water in the vessel.
A related object of the present invention is to provide a method of using oxidation reduction measurement technology to control the residual reducing agent concentration level in wastewater discharge from the treatment plant.
It is an additional object of the present invention to provide a method of operatively controlling the addition of the reducing agent in response to the measured concentration levels of the reducing agent residual in the wastewater after addition of the reducing agent.
It is a still further object of the present invention to provide a method for reducing agent residual control which utilizes a measuring device which is sensitive to low concentration levels of the reducing agent in the wastewater.